Zimbabwean opposition parties will on Saturday hold protests throughout the country with or without police clearance to press the election agency for free elections in 2018, officials said, weeks after another planned demonstration turned violent.

Police on August 26 fired tear gas and water cannon at opposition leaders and hundreds of demonstrators at an anti-government protest, before unrest swept across large parts of the capital Harare.

Eighteen opposition parties under the National Election Reform Agenda (NERA) say the Zimbabwe Electoral Commission is biased in favor of President Robert Mugabe's ruling ZANU-PF and is run by security agents loyal to Mugabe, charges the agency denies.

The parties want the next vote in 2018 to be supervised by international observers, including the United Nations and seek changes in election laws they say hinder a free and fair vote.

"We are going ahead with the demonstration with or without their (police) blessing. We don't need their permission, we have the law in our favor, we are sure we are right," NERA official and Movement for Democratic Change secretary general Douglas Mwonzora told reporters.

Police spokeswoman Charity Charamba was not reachable for comment.

Under Zimbabwe's security laws, protest organizers only need to notify the police of their plans, but law enforcement agents routinely refuse to sanction opposition demonstrations.

Saturday's protest will come after the High Court last week struck down a two-week ban on public protests that had been issued by the police in the capital Harare. - Reuters